Tuesday, 24 September 2013

Comments on Antartic sea ice

If the Southern Ocean is warming, why is sea ice increasing? There are several contributing factors. One is the drop in ozone levels over Antarctica. The hole in the ozone layer above the South Pole has caused cooling in the stratosphere (Gillet 2003). A side-effect is a strengthening of the cyclonic winds that circle the Antarctic continent (Thompson 2002). The wind pushes sea ice around, creating areas of open water known as polynyas. More polynyas leads to increased sea ice production (Turner 2009).

Another contributor is changes in ocean circulation. The Southern Ocean consists of a layer of cold water near the surface and a layer of warmer water below. Water from the warmer layer rises up to the surface, melting sea ice. However, as air temperatures warm, the amount of rain and snowfall also increases. This freshens the surface waters, leading to a surface layer less dense than the saltier, warmer water below. The layers become more stratified and mix less. Less heat is transported upwards from the deeper, warmer layer. Hence less sea ice is melted (Zhang 2007).

There is a trend for sea ice around Antarctica to be increasing in extent, though this is not of the same order of magnitude as what is going on in the Arctic. It certainly doesn’t mean they somehow balance out and everything is fine, we can keep on emitting carbon. Most of the ice in Antarctica is in fact on land, and the volume of that ice is decreasing. If you look at all of the data rather than isolate selective facts, the overall picture is something everyone should be concerned about.

Always amuses me how some among the Grumpy True Disbeliever demographic have issues understanding that the dynamics of sea ice formation may be different between the Arctic and Antarctic.

One has a thin layer of ice over an pretty well contained deep sea. The other has a layer of ice around a large continent surrounded by a circulating ocean that is generally covered in a layer of ice up to 3km thick. But they should both behave the same Wot??